1. Field of the Invention
The present invention relates generally to the field of radiation sensitive resist films used in microcircuit fabrication and, more particularly, to the fabrication of positive electron-beam sensitive resist films comprised of copolymers of methacrylic acid and methacrylonitrile having high sensitivity, high plasma etch resistance and submicron resolution.
2. Description of the Prior Art
The application of electron beam techniques to the art of semiconductor fabrication has enabled great strides to be made in reducing the minimum line width and thus the size of an integrated circuit pattern which can successfully be manufactured. This has been accomplished both through progress in the technology involved with the perfection in the precision of the electron beam system itself and in the progress which has been made in the technology concerned with the pattern defining medium or resist material.
In electron beam microfabrication, a substrate, which may be, for example, silicon dioxide, silicon, glass or chromium plated glass, is further coated with a layer of polymer resist material and the resist is patterned by changing the solubility of the polymer with an electron beam. Subsequently, the mask is "developed" by dissolving away the unwanted area of polymer utilizing a suitable solvent material and the resultant pattern is used as a mask either for plating, chemical etching, ion etching or ion implantation. This electron-beam lithography (EBL) is an integrated circuit production technique which utilizes a polymer resist to delineate circuit patterns for monolithic circuits.
When polymers of the required type are irradiated with an electron beam, the molecular structure is affected such that some of the polymer molecules are excited or ionized by the beam. This excitation causes some of the resist molecules to cross link with other molecules in the polymer and others to degrade or undergo scission. The predominant manner in which such a polymeric material reacts to exposure to an electron beam has led electron resists to be classified into two main categories. Thus, a polymer which becomes predominantly gelled or cross-linked, and thereby decreases its solubility after irradiation, is termed a negative resist. Conversely, if the scission process predominates and the solubility of the polymer increases after irradiation, it is called a positive resist. The resists of the present invention are positive resists.
A suitable electron resist must have various physical and chemical properties which allow it to fulfill the requirements for electron beam fabrication. The polymer material involved must be sensitive to an electron current of a fairly low value or the resist sensitivity itself will be the limiting factor on the writing speed and line width which can be achieved. The resist medium must be capable of a high resolution or resist contrast compatible with that achieved in writing and etching techniques utilizing the high resolution capability available with electron beam technology. The resist must also have the ability to adhere satisfactorily to a variety of substrates used in different microfabrication applications. The medium also must be able to withstand normal acid, base, and ion etching processes and should not be sensitive to small daily process variabilities.
In the prior art it is known to produce positive type electron resist films of comonomers of methylmethacrylate (MMA) with such monomers as acrylonitrile (AN), methacrylonitrile (MCN) and maleic anhydride. Such combinations are shown in U.S. Pat. No. 3,914,462 to Morishita et al and issued Oct. 21, 1975. Similar copolymer materials are disclosed in a patented Gipstein et al U.S. Pat. No. 4,011,351 issued Mar. 8, 1977 which discloses a method of producing a positive resist image from copolymers of alkyl mathacrylate units and polymerized units of certain other ethylenically unsaturated monomers. Those copolymers include alkyl mathacrylate units wherein the alkyl group contains from one to four carbon atoms copolymerized with ethylenically unsaturated monomers which may contain any of numerous substituted groups or multipally substituted groups. Certain other polymerized alkyl mathacrylate copolymers including polymerized monoethylenically unsaturated acid units have also been proposed in a patent to Feder et al, U.S. Pat. No. 3,984,582 issued Oct. 5, 1976.
The Feder et al patent, cited above, also discloses a prebaking step in which the prepared polymeric resist, having been coated upon a substrate by spin coating or dipping and dried to remove volatile material, is prebaked. The prebaking occurs either in air or in a vacuum at a temperature above the glass transition temperature of the polymeric material, but below the decomposition temperature of the material to remove any remaining solvent which, in the case of poly (MMA-co-MAA) is preferably in the 160.degree. to 220.degree. C. range. The prebaked polymeric resist is then exposed to an electron beam image and subsequently developed. The prebaking at temperatures of 160.degree. C. to 220.degree. C. changes the solubility of the copolymer and improves adhesion to the substrate.
The change in solubility of copolymer resists such as MMA/MAA has been found to be highly dependent on the prebaking temperature and time and is very hard to control. Therefore, the temperature and time control in the prebake step must be extremely accurate in order to maintain repeatability of resist sensitivity and resolution in the development of such resists.
A later patent, U.S. Pat. No. 4,087,569, issued May 2, 1978 to Hatzakis also discusses the prebaking of poly (MMA-co-MAA). Hatzakis emphasizes the drawback in Feder et al associated with the requirement of very precise temperature and time control in the baking of copolymer resists in film form after being coated on the work piece or substrate. He further points out that baking of a polymeric film on a work piece causes cross-linking, etc. which results in producing insoluable particles which may leave an insoluable residue on the substrate when the exposed areas are developed with a strong solvent such as ethyl cellosolve acetate (ECA).
Hatzakis, on the contrary, teaches the step of prebaking the powder form of the radiation-sensitive polymeric material poly (MMA-co-MAA) at a temperature below the decomposition temperature, prior to dissolving the copolymer in a solvent and coating it on the work piece. The copolymer resist film on the work piece may also be post-baked but only to dry the film and evaporate the coating solvent. In that patent, the baking step on the coated work piece is not utilized to change any of the properties of the polymer itself.
Thus, it can be seen from the prior art that numerous polymers including copolymers of MAA and substituted MAA esters copolymerized with various ethylenically unsaturated monomers have been proposed. In addition, various baking steps have been utilized in the preparation of resist films utilizing such copolymer combinations as MMA/MAA with varying degrees of success, and it appears that much confusion exists as to where in the process the baking step should be used.
In addtion to the requirements normally associated with prior art positive electron resists, there has been an increasing trend in integrated circuit fabrication toward the use of dry etching techniques, especially plasma etching as a method of using the developed exposed resist masks for integrated circuit fabrication. The compatibility of a resist material with the plasma etching process has therefore also become an important consideration in the selection of the resist. In the case of a positive resist, the resist material must be etch resistant such that the area of the pattern exposed to radiation are etched in preference to those not exposed.